The following methods have been proposed as methods for removing impurities contained in a polycarbonate resin, such as organic impurities, inorganic impurities, solvents, additives, monomers, oligomers, etc.
(1) A method which comprises subjecting a powdery polycarbonate resin to extraction with a mixed solvent of, for example, methylene chloride and toluene as disclosed in, for example, Japanese Patent Publication Nos. 38-16347 and 48-6620;
(2) A method which comprises contacting a polycarbonate resin solution with active carbon as disclosed in, for example, Japanese Patent Publication No. 42-2915;
(3) A method which comprises extracting a polycarbonate resin solution with an aqueous alkaline solution as disclosed in, for example Japanese Patent Publication No. 42-2915; and
(4) A method which comprises stirring a molten polycarbonate resin under high vacuum as disclosed in, for example, Japanese Patent Publication No. 42-16079.
However, those methods have the following disadvantages.
In the methods (1), (2) and (3) above, since solvents are used for the purification, contamination of the purified products with the residual solvents and the impurities in the solvents cannot be avoided completely. Further, the application of those three techniques is limited to the purification of polycarbonate resins which are in the stage of powders or earlier stages, and those methods are utterly ineffective in the purification of pellets or formed articles from which the impurities resulting from thermal decomposition in both extrusion and molding and the additives, such as a stabilizer and a releasing agent, which become unnecessary after molding and should be treated as impurities, should be removed. Further, with respect to their purification effects, the methods (1) and (2) are ineffective in the removal of inorganic impurities although effective against organic impurities, monomers and oligomers, while the method (3) can remove substantially monomers only and its application is limited to partial purification.
Moreover, those three methods have another disadvantage that the post-treatments or regeneration treatments of the mixed solvents, active carbon and aqueous alkaline solutions which are discharged after purification, are troublesome and expensive.
The method (4) above is effective in removing solvents, organic impurities and monomers, but unable to remove nonvolatile impurities such as inorganic impurities and oligomers, and also has a defect of quality deterioration of the resins due to heating. Therefore, the method (4) is disadvantageous from the standpoint of a molded article.
Therefore, no method is known at present which can remove various impurities contained in a polycarbonate resin in the stage of solid resins, with a one-step treatment at a high efficiency. Further, from the standpoint of a molded article which is an objective product, no any method is known at present which can remove the impurities without substantially impairing the shape, the surface state, and the like of the molded article.